LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rate and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	$ 3,833,820		$ 3,418,445
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,403,725		1,195,058
Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	167,524		137,153
Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	411,006		351,258
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	723,964		682,169
Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	238,499		228,291
One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	539,894		529,494
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	349,208		295,022
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	3,737,037	[1]	3,308,889	[2]
Pass (Risk Ratings 1-5) [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,375,885	[1]	1,160,921	[2]
Pass (Risk Ratings 1-5) [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	166,712	[1]	131,523	[2]
Pass (Risk Ratings 1-5) [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	395,356	[1]	332,150	[2]
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	691,143	[1]	655,007	[2]
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	115,000		94,000
Pass (Risk Ratings 1-5) [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	234,101	[1]	225,329	[2]
Pass (Risk Ratings 1-5) [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	527,384	[1]	511,967	[2]
Pass (Risk Ratings 1-5) [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	346,456	[1]	291,992	[2]
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	32,428		18,115
Special Mention [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	3,717		6,614
Special Mention [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Special Mention [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		350
Special Mention [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	27,453		10,484
Special Mention [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,055		561
Special Mention [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	63		0
Special Mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	140		106
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	64,344		90,888
Substandard [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	24,123		26,979
Substandard [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	812		5,630
Substandard [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	15,650		18,758
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	5,368		16,669
Substandard [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	3,343		2,401
Substandard [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	12,447		17,527
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	2,601		2,924
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	11		553
Doubtful [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		544
Doubtful [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		9
Doubtful [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	11		0
Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	3,817,084		3,393,677
Performing loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,402,328		1,188,771
Performing loans [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	167,524		137,153
Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	409,731		350,065
Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	723,427		681,445
Performing loans [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	236,902		228,187
Performing loans [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	529,292		514,351
Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	347,880		293,705
Nonperforming loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	16,736	[3]	24,768	[3]
Nonperforming loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,397	[3]	6,287	[3]
Nonperforming loans [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0	[3]	0	[3]
Nonperforming loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,275	[3]	1,193	[3]
Nonperforming loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	537	[3]	724	[3]
Nonperforming loans [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,597	[3]	104	[3]
Nonperforming loans [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	10,602	[3]	15,143	[3]
Nonperforming loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	$ 1,328	[3]	$ 1,317	[3]

[1] The Pass category includes some performing loans that are part of homogenous pools which are not individually risk-rated. This includes all consumer loans, all one- to four-family residential loans and, as of December 31, 2014, in the commercial business category, $115 million of credit-scored small business loans. As loans in these pools become non-performing, they are individually risk-rated.

[2] The Pass category includes some performing loans that are part of homogenous pools which are not individually risk-rated. This includes all consumer loans, all one- to four-family residential loans and, as of December 31, 2013, in the commercial business category, $94 million of credit-scored small business loans. As loans in these pools become non-performing, they are individually risk-rated.

[3] Non-performing loans include loans on non-accrual status and loans more than 90 days delinquent, but still accruing interest.